The invention relates generally to field emission devices and manufacturing methods thereof; and more particularly to a carbon nanotube field emission device and a manufacturing method thereof.
Field emission materials are used in a variety of applications such as, for example, flat panel displays to emit electrons. Typical field emission materials are metals such as, for example, molybdenum (Mo), tantalum (Ta), silicon (Si), and diamond (C). However, such materials need high emission voltages to emit electrons, and cannot carry high electric currents reliably. Carbon nanotubes typically have superior performance; in particular, good electron emission capability at low emission voltages, generally less than 100 volts. Furthermore, carbon nanotubes can carry high electric currents reliably. Due to these properties, carbon nanotubes are considered to be an ideal field emission material for application in a variety of display devices, including flat panel displays such as field emission displays.
Carbon nanotube field emission devices typically include a substrate acting as a cathode plate and a carbon nanotube array formed on the substrate and acting as emitters. Methods for forming the carbon nanotube array on the substrate typically include steps of coating, printing or growing. The printing process is typically performed as follows. Firstly, the carbon nanotubes are mixed in a plasma. Secondly, the mixture is printed on the substrate. Such a printing process has the following disadvantages. The carbon nanotubes are mixed in the plasma and thus cannot contact the substrate directly. Therefore, the electrical contact between the carbon nanotubes and the substrate is bad. The typical result of such bad contact is that the carbon nanotube field emission device cannot emit electrons uniformly and stably.
The growing process is typically performed as follows. Firstly, a catalyst film is deposited on a substrate. Secondly, a carbon nanotube array is grown on the substrate by chemical vapor deposition (CVD). U.S. Pat. No. 6,232,706 discloses a field emission device and a method for manufacturing the field emission device. The field emission device includes a substrate, a catalyst material deposited on a porous surface of the substrate, and one or more bundles of parallel carbon nanotubes extending from the catalyst material in a direction substantially perpendicular to the substrate. The method includes the steps of: (a) depositing a catalyst material on the porous surface of the substrate; and (b) exposing the catalyst material to a carbon containing gas at an elevated temperature such that one or more bundles of parallel carbon nanotubes grow on the substrate. The carbon nanotubes grown on the substrate by this method have good electrical contact with the substrate. However, the carbon nanotubes are not secured on the substrate. Thus, the carbon nanotubes are apt to be pulled out from the substrate by electric field force in a strong electric field.
What is needed, therefore, is a carbon nanotube field emission device that is secured on a substrate and well-connected electrically with the substrate, and that can emit electrons uniformly and stably.
What is also needed is a method for manufacturing the above-described carbon nanotube field emission device.